Modern NMR apparatus is most often characterized in gross by cylindrical symmetry. A typical NMR magnet is of the superconducting variety and is housed in a dewar which includes a room temperature cylindrical bore in which a very carefully controlled homogeneous magnetic field is sustained by operation of the superconducting magnet in the interior of the dewar. The NMR probe contains the excitation and detection interface to the sample and is mounted in a cylindrical housing for insertion in the bore.
The basic requirement of field uniformity is painstakingly achieved in a process of field shimming whereby irregularities of the resulting magnetic field are compensated. The NMR apparatus for certain application is capable of precision of the order of one part in 10.sup.9. Consequently many apparently minor effects are readily observable. For example, any object introduced into the sensitive volume portion of the magnet bore will disturb the magnetic field distribution if the object has a non zero magnetic susceptibility. Further shimming of the magnet may be required to compensate this disturbance. Consequently the NMR probe which is inserted into this region must be capable of reoccupying the exact region in the same orientation for which a careful shimming operation has previously been completed.
The NMR probe is designed to exhibit gross cylindrical symmetry, but the internal structure of the probe contains circuits, circuit elements, connectors, fluid supply tubes, mechanical mounting means for all of the above and the like. Thus the internal structure generally exhibits no geometric symmetry and the azimuthal orientation of the probe in the bore results in a particular distribution of probe components in the magnet bore. Moreover, these components may exhibit diverse magnetic susceptibility properties. It is therefore desirable to assure that the location and orientation of the probe is reproducible in NMR apparatus in order to avoid any requirement for additional shimming operations.
In the prior art it was common to obtain a reproducible positioning of the probe in the z coordinate (that is, the axis of the bore) by the straightforward step of providing a stop, or equivalent, against which the probe body could be urged. Other prior art provided for a pair (or triplet) of floating thumbscrews to secure a collar about the probe body to a flange of the magnet cryostat. In this arrangement the tolerance in azimuthal orientation is undesirable and the precision of reproducible angular position is lacking. In the present invention both the axial and azimuthal positioning is obtained in an arrangement which mechanically assures positive positioning reproducibility, or alternatively repositioning at selected angular increments of azimuthal angle from a known desired orientation.
In other prior art pertaining to magnetic resonance imaging, patient support mechanisms are employed for the purpose of transporting the subject to a known position in the magnetic field space, whereas the RF excitation and detection interface is not disturbed. In typical apparatus where the NMR probe is removable, the sample is independently supported.
A preferred arrangement of the present invention employs a bayonet pin mounting arrangement to secure the body of the probe to a mounting collar affixed to the magnet cryostat. Further to this embodiment, the pinning arrangement may be provided in selected symmetry to permit selected known discrete angular positioning which is useful for samples which are cylindrically asymmetric.